The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An ever-increasing number of relatively cheap, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in the IEEE 802.11 Standard, including for example, the IEEE 802.11a (1999) Standard and its updates and amendments, the IEEE 802.11g (2003) Standard, as well as the IEEE 802.11n Standard, all of which are collectively incorporated herein fully by reference. These standards have been or are in the process of being commercialized with the promise of 54 Mbps or more effective bandwidth, making them a strong competitor to traditional wired Ethernet and the more ubiquitous “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard.
Generally speaking, transmission systems compliant with the IEEE 802.11a and 802.11g Standards or “802.11a/g” as well as the IEEE 802.11n Standard achieve their high data transmission rates using Orthogonal Frequency Division Modulation or OFDM encoded symbols mapped up to a 64 quadrature amplitude modulation (QAM) multi-carrier constellation. Generally speaking, the use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective sub-carrier or tone upon which data may be modulated. Thus, each frequency sub-band of the OFDM system may be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system.
Generally, transmitters used in the wireless communication systems that are compliant with the aforementioned IEEE 802.11a/802.11g/802.11n Standards as well as other standards such as the IEEE 802.16a/e/j/m Standard, perform multi-carrier OFDM symbol encoding (which may include error correction encoding and interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers, resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR).
Likewise, the receivers used in the wireless communication systems that are compliant with the aforementioned IEEE 802.11a/802.11g/802.11n Standards and the IEEE 802.16a/e/j/m Standards generally include an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that processes the OFDM encoded symbols bearing the data of interest. Generally, the digital form of each OFDM symbol presented in the frequency domain is recovered after baseband downconversion, conventional analog to digital conversion and Fast Fourier Transformation of the received time domain analog signal. Thereafter, the baseband processor performs frequency domain equalization (FEQ) and demodulation to recover the transmitted symbols. The recovered and recognized stream of symbols is then decoded, which may include deinterleaving and error correction using any of a number of known error correction techniques, to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter.
Similarly, in a single carrier communication system, such as the IEEE 802.11b Standard, a transmitter performs symbol encoding (which may include error correction encoding and interleaving), digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers. A receiver in a single carrier communication system includes an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that demodulates and decodes (which may include deinterleaving and error correction) the encoded symbols to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter.
In wireless communication systems, the RF modulated signals generated by the transmitter may reach a particular receiver via a number of different propagation paths, the characteristics of which typically change over time due to the phenomena of multi-path and fading. Moreover, the characteristics of a propagation channel differ or vary based on the frequency of propagation. To compensate for the time varying, frequency selective nature of the propagation effects, and generally to enhance effective encoding and modulation in a wireless communication system, each receiver of the wireless communication system may periodically develop or collect channel state information (CSI) for each of the frequency channels, such as the channels associated with each of the OFDM sub-bands discussed above. Generally speaking, CSI is information defining or describing one or more characteristics about each of the OFDM channels (for example, the gain, the phase and the signal-to-noise ratio (SNR) of each channel). In a single carrier communication system, information similar to CSI information may be developed by the receiver. Upon determining the CSI for one or more channels, the receiver may send this CSI back to the transmitter, which may use the CSI for each channel to precondition the signals transmitted using that channel so as to compensate for the varying propagation effects of each of the channels.
An important part of a wireless communication system is therefore the selection of the appropriate data rates, and the coding and modulation schemes to be used for a data transmission based on channel conditions. Generally speaking, it is desirable to use the selection process to maximize throughput while meeting certain quality objectives, such as those defined by a desired frame error rate (FER), latency criteria, etc.
To further increase the number of signals which may be propagated in the communication system and/or to compensate for deleterious effects associated with the various propagation paths, and to thereby improve transmission performance, it is known to use multiple transmit and receive antennas within a wireless transmission system. Such a system is commonly referred to as a multiple-input, multiple-output (MIMO) wireless transmission system and is specifically provided for within the IEEE 802.11n Standard. Generally speaking, the use of MIMO technology produces significant increases in spectral efficiency and link reliability, and these benefits generally increase as the numbers of transmit and receive antennas within the MIMO system increases.
In addition to the frequency channels created by the use of OFDM, for example, a MIMO channel formed by the various transmit and receive antennas between a particular transmitter and a particular receiver includes a number of independent spatial channels. As is known, a wireless MIMO communication system can provide improved performance (e.g., increased transmission capacity) by utilizing the additional dimensionalities created by these spatial channels for the transmission of additional data. Of course, the spatial channels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multi-path effects) across the overall system bandwidth and may therefore achieve different SNRs at different frequencies (i.e., at the different OFDM frequency sub-bands) of the overall system bandwidth. Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted using the different frequency sub-bands of each spatial channel for a particular level of performance may differ from frequency sub-band to frequency sub-band.
However, instead of using the various different transmit and receive antennas to form separate spatial channels on which additional information is sent, better transmission and reception properties can be obtained in a MIMO system by using each of the various transmit antennas of the MIMO system to transmit the same signal while phasing (and amplifying) the signal as the signal is provided to the various transmit antennas to achieve beamforming, also sometimes referred to as beamsteering. Generally speaking, beamforming creates a spatial gain pattern having one or more high gain lobes or beams (as compared to the gain obtained by an omni-directional antenna) in one or more particular directions, while reducing the gain over that obtained by an omni-directional antenna in other directions. If the gain pattern is configured to produce a high gain lobe in the direction of each of the receiver antennas, the MIMO system can obtain better transmission reliability between a particular transmitter and a particular receiver, over that obtained by single transmitter-antenna/receiver-antenna systems.
There are many known techniques for determining a steering matrix specifying the beamforming coefficients that need to be used to properly condition the signals being applied to the various transmit antennas so as to produce the desired transmit gain pattern at the transmitter. As is known, these coefficients may specify the gain and phasing of the signals to be provided to the transmitter antennas to produce high gain lobes in particular or predetermined directions. These techniques include, for example, transmit-MRC (maximum ratio combining) and singular value decomposition (SVD).
An important part of determining the steering matrix is taking into account the specifics of the channel between the transmitter and the receiver, referred to herein as the forward channel. As a result, steering matrices are typically determined based on the CSI of the channel. To determine the CSI or other specifics of a forward channel, the transmitter may first send a known test or calibration signal to the receiver. The receiver may then compute or determine the specifics of the forward channel (e.g., the CSI for the forward channel), and may send the CSI or other indications of the forward channel back to the transmitter, thereby requiring signals to be sent both from the transmitter to the receiver and then from the receiver back to the transmitter in order to perform beamforming in the forward channel. This exchange typically may occur each time the forward channel is determined (e.g., each time a steering matrix is to be calculated for the forward channel).
Determining a steering matrix based on the CSI (or other information) of the forward channel using the process described above is often referred to as explicit beamforming. To reduce the amount of startup exchanges required to perform explicit beamforming, it is known to perform another process often referred to as implicit beamforming. With implicit beamforming, the steering matrix is calculated or determined based on the assumption that the forward channel (i.e., the channel from the transmitter to the receiver in which beamforming is to be accomplished) can be estimated from the reverse channel (i.e., the channel from the receiver to the transmitter). In particular, the forward channel can ideally be estimated as the matrix transpose of the reverse channel matrix. Thus, in the ideal case, the transmitter only needs to receive signals from the receiver to produce a steering matrix for the forward channel, as the transmitter can use the signals from the receiver to determine the reverse channel, and can simply estimate the forward channel as a matrix transpose of the reverse channel matrix. As a result, implicit beamforming reduces the amount of startup exchange signals that need to be sent between a transmitter and a receiver because the transmitter can estimate the forward channel based solely on signals sent from the receiver to the transmitter.
Either explicit or implicit beamforming may be used to determine a steering matrix or multiple steering matrices. In an OFDM system, each sub-carrier or tone (or each of a plurality of groups of sub-carriers or tones) may have an associated steering matrix. The multiple steering matrices in the OFDM system may be used by the system to determine output power levels for power amplifiers (PAs) in an attempt to operate corresponding transmit antennas at a maximized power while simultaneously minimizing distortion. Typically, the system may vary the energy of an input to a PA and/or adjust an output power level of the PA based on an average calculated power for the transmit antenna. Unfortunately, this averaging approach may result in distortion or performance loss due to nonlinearity at the power amplifier associated with the transmit antenna.
Moreover, in some instances, such as when the system transitions to using a different modulation coding scheme with a different number of spatial streams, the power adjustments must be re-calculated as the total calculated power for each transmit antenna may change. This re-calculation is computationally expensive.